Copolymer, organic solar cell using the same and manufacturing method thereof

ABSTRACT

Disclosed herein are a 3,6-carbazole-containing copolymer, an organic solar cell comprising the copolymer in an organic material layer including a photoactive layer, and a method for fabricating the organic solar cell.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The present disclosure relates to a novel copolymer, an organic solar cell comprising the same and a method for fabricating the organic solar cell.

2. Description of the Prior Art

Since the possibility of solar cells based on organic polymers was first shown by Heeger at the University of California, Santa Barbara (UCSB), in 1992, there have been many studies thereon. Such solar cells include heterojunction thin film devices comprising a mixture of an organic polymer, which absorbs light, with a C60 or C70 fullerene derivative which has a very high electron affinity. These heterojunction thin film devices comprise a transparent positive electrode made of indium tin oxide (ITO) and a negative electrode made of a metal such as Al, which has a low work function.

The photoactive layer comprising the organic polymer absorbs light to form electron-hole pairs or excitons. The electron-hole pairs move to the interface between the copolymer and the C60 or C70 fullerene derivative at which they are separated into electrons and holes. Then, the electrons move to the metal electrode, and the holes move to the transparent electrode, thereby generating electrodes.

Currently, the efficiency of organic polymer thin-film solar cells based on organic polymers reaches 7-8% (Nature Photonics, 2009, 3, 649-653).

However, the efficiency of the organic polymer thin-film solar cells is still low compared to the maximum efficiency of solar cells based on silicon (about 39%). Thus, the development of organic polymer solar cells having a higher efficiency is required.

Korean Patent Laid-Open Publication No. 10-2010-0111767 discloses a conductive polymer comprising 2,7-carbazole in the main chain and an organic solar cell comprising the same. According to the disclosure of the above patent publication, the conductive polymer comprising 2,7-carbazole in the main chain improves light absorption and hole mobility to improve the efficiency of the solar cell.

However, there is a problem in that it is not easy to improve optical efficiency, because the mobility of holes is relatively lower than the mobility of electrons, despite the use of 2,7-carbazole.

PRIOR ART DOCUMENTS

[Patent Documents]

-   Korean Patent Laid-Open Publication No. 10-2010-0111767

[Non-Patent Documents]

-   Macromolecules 2011, 44(7), 1909-1919 -   Nature Photonics, 2009, 3, 649-653 -   B. Nicolas, M. Alexandre, M. Leclerc, Adv. Mater. 19, 2295-2300 -   Y. Human, A. Solyman, I. Ahmed, W. C. Darren, K. James, L. G.     David, J. Mater. Chem. 21, 13649-13656

SUMMARY OF THE DISCLOSURE

It is an object of the present disclosure to provide an organic semiconductor material, which can exhibit excellent electrical properties thanks to high hole mobility and has high photovoltaic conversion efficiency, and an organic solar cell comprising the same.

To achieve the above object, in one aspect, the present disclosure provides a copolymer comprising: a first unit represented by the following formula 1; a second unit represented by the following formula 2; and a third unit represented by the following formula 3:

wherein o, p, q and r are each an integer ranging from 0 to 3; s and t are each an integer ranging from 0 to 4; R_(a) to R_(d) are the same or different and are each independently selected from the group consisting of hydrogen, deuterium, a halogen group, a nitrile group, a nitro group, an imide group, an amide group, a hydroxyl group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted aryloxy group, a substituted or unsubstituted alkylthioxy group, a substituted or unsubstituted arylthioxy group, a substituted or unsubstituted alkylsulfoxy group, a substituted or unsubstituted arylsulfoxy group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted silyl group, a substituted or unsubstituted boron group, a substituted or unsubstituted alkylamine group, a substituted or unsubstituted aralkylamine group, a substituted or unsubstituted arylamine group, a substituted or unsubstituted heteroarylamine group, a substituted or unsubstituted aryl group, a substituted or unsubstituted fluorenyl group, a substituted or unsubstituted carbazole group, and a substituted or unsubstituted heterocyclic group containing at least one heteroatom selected from among N, O and S, or adjacent two of R_(a) to R_(d) may form a condensed ring; and R₁ to R₇ are the same or different and are each independently selected from the group consisting of hydrogen, deuterium, a halogen group, a nitrile group, a nitro group, an imide group, an amide group, a hydroxyl group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted aryloxy group, a substituted or unsubstituted alkylthioxy group, a substituted or unsubstituted arylthioxy group, a substituted or unsubstituted alkylsulfoxy group, a substituted or unsubstituted arylsulfoxy group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted silyl group, a substituted or unsubstituted boron group, a substituted or unsubstituted alkylamine group, a substituted or unsubstituted aralkylamine group, a substituted or unsubstituted arylamine group, a substituted or unsubstituted heteroarylamine group, a substituted or unsubstituted aryl group, a substituted or unsubstituted fluorenyl group, a substituted or unsubstituted carbazole group, and a substituted or unsubstituted heterocyclic group containing at least one heteroatom selected from among N, O and S, or adjacent two of R₁ to R₇ may form a condensed ring.

In another aspect, the present disclosure provides an organic solar cell comprising: a first electrode; a second electrode opposite the first electrode; one or more organic material layers interposed between the first and second electrodes and including a photoactive layer, wherein one or more of the organic material layers comprises the copolymer comprising the first unit, second unit and third unit represented by formulas 1 to 3, respectively.

In still another aspect, the present disclosure provides a method for fabricating an organic solar cell, the method comprising the steps of: preparing a substrate; forming a first electrode on a region of rear sides of the substrate; forming on the first electrode an organic material layer comprising the copolymer including the first unit of formula 1, the second unit of formula 2 and the third unit of formula 3; and forming a second electrode on the organic material layer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graphic diagram showing a comparison of charge mobility between 3,6-carbazole-containing copolymers and 2,7-carbazole-containing copolymers.

FIG. 2 is a graphic diagram showing current-voltage curves for organic solar cells of Examples 2 to 4 and Comparative Examples 1 and 2.

FIG. 3 is a graphic diagram showing the mobilities of holes and electrons in a device comprising a polymer of Comparative Example 1.

FIG. 4 is a graphic diagram showing the mobilities of holes and electrons in a device comprising a polymer of Example 2.

FIG. 5 is a graphic diagram showing the mobilities of holes and electrons in a device comprising a polymer of Example 3.

FIG. 6 is a graphic diagram showing the mobilities of holes and electrons in a device comprising a polymer of Example 4.

DETAILED DESCRIPTION OF THE DISCLOSURE

Hereinafter, the present disclosure will be described in detail.

The present disclosure provides a copolymer comprising a first unit represented by formula 1, a second unit represented by formula 2, and a third unit represented by formula 3.

As used herein, the term “unit” means the polymerized form of a repeating monomer unit in the copolymer.

The first unit of formula 1 contains a 3,6-carbazole group. An organic solar cell comprising the copolymer comprising the first unit has high charge mobility compared to a solar cell comprising a 2,7-carbazole-containing copolymer, and the first unit of formula 1 contributes to the stability of the copolymer.

FIG. 1 is a graphic diagram showing a comparison of charge mobility between 3,6-carbazole-containing copolymers and 2,7-carbazole-containing copolymers.

In one embodiment of the present disclosure, the first unit of formula 1 is preferably present in an amount of greater than 0 mole % and 45 mole % or less based on the total moles of monomers constituting the copolymer.

In another embodiment, the content of the first unit of formula 1 is 0.1-30 mole % based on the total moles of the monomers. In still another embodiment, the content of the first unit of formula 1 is 0.5-20 mole % based on the total moles of the monomers. In yet another embodiment, the content of the first unit of formula 1 is 0.5-15 mole % based on the total moles of the monomers. In another further embodiment, the content of the first unit of formula 1 is 1-10 mole % based on the total moles of the monomers.

In the 3,6-carbazole-containing copolymer, a nitrogen atom is conjugated to the main chain to stabilize holes and increase the mobilities of holes and charges, so that hole mobility and electron mobility in the organic solar cell comprising the copolymer can be balanced. Thus, the present disclosure provides an organic semiconductor material, which can exhibit excellent electrical properties thanks to the balance between hole mobility and electron mobility and has high photovoltaic conversion efficiency, and an organic solar cell comprising the same.

Conventional 2,7-carbazole-containing copolymers do not appear to exhibit excellent electrical properties, because no nitrogen atom is not conjugated to the main chain.

In contrast, if the content of the first unit of formula 1 in the copolymer comprising the first unit of formula 1 is 45 mole % or less based on the total moles of the monomers of the copolymer, a reduction in the optical absorption of the copolymer, which results from the relatively low absorbance of the carbazole group, can be minimized. In addition, in this case, a copolymer having a suitable molecular weight can be prepared.

Examples of the substituents of the above formulas will be described below, but are not limited thereto.

In the present disclosure, the alkyl group may be a straight or branched-chain alkyl group, and the number of carbon atoms thereof is preferably 1 to 20, but is not limited thereto. Specific examples of the alkyl group include, but are not limited to, methyl, ethyl, propyl, isopropyl, butyl, t-butyl, pentyl, hexyl and heptyl groups.

In the present disclosure, the alkenyl group may be a straight or branched-chain alkenyl group, and the number of carbon atoms thereof is preferably 2 to 40, but is not limited thereto. Specific examples of the alkenyl group include, but are not limited to, aryl-substituted alkenyl groups, such as stylbenzyl and styrenyl groups.

In the present disclosure, the alkoxy group may be a straight, branched or cyclic chain alkoxy group. The number of carbon atoms of the alkoxy group is preferably 1 to 25, but is not limited thereto. Specific examples of the alkoxy group include, but are not limited to, methoxy, ethoxy, n-propyloxy, iso-propyloxy, n-butyloxy and cyclopentyloxy groups.

In the present disclosure, the cycloalkyl group preferably contains 3 to 60 carbon atoms, but is not limited thereto. Particularly preferred examples of the cycloalkyl group include cyclopentyl and cyclohexyl groups.

In the present disclosure, the halogen group may be fluorine, chlorine, bromine or iodine.

In the present disclosure, the aryl group may be monocyclic, and the number of carbon atoms thereof is preferably 6 to 60, but is not limited thereto. Specific examples of the aryl group include, but are not limited to, monocyclic aromatic groups, such as phenyl, biphenyl, triphenyl, terphenyl or stilbene groups, and polycyclic aromatic groups, such as naphthyl, anthracenyl, phenanthrenyl, pyrenyl, perylenyl, tetracenyl, chrysenyl, fluorenyl, acenaphthacenyl, triphenylene or fluoranthene groups.

In the present disclosure, the heterocyclic group is a heterocyclic group containing at least one heteroatom selected from among O, N and S, and the number of carbon atoms thereof is preferably 2 to 60, but is not limited thereto. Examples of the heterocyclic group include, but are not limited to, thiophene, furan, pyrrol, imidazole, thiazole, oxazole, oxadiazole, triazole, pyridyl, bipyridyl, triazine, acridyl, pyridazine, quinolinyl, isoquinolinyl, indole, carbazole, benzoxazole, benzothiazole, benzimidazole, benzocarbazole, benzothiophene, dibenzothiophene, benzofuranyl, phenanthroline and dibenzofuranyl groups.

In the present disclosure, the imide group preferably contains 1 to 25 carbon atoms, but is not limited thereto. Specific examples of the imide group include, but are not limited to, compounds having the following formulas:

In the present disclosure, the amide group is an amide group whose nitrogen may be mono- or di-substituted with hydrogen, a straight, branched or cyclic chain alkyl group containing 1 to 25 carbon atoms or an aryl group containing 6 to 25 carbon atoms. Specific examples of the amide group include, but are not limited to, compounds having the following formulas:

In the present disclosure, the ester group is an ester group whose oxygen may be substituted with a straight, branched or cyclic chain alkyl group containing 1 to 25 carbon atoms or an aryl group containing 6 to 25 carbon atoms. Specific examples of the ester group include, but are not limited to, compounds having the following formulas:

In the present disclosure, the heteroaryl group may be selected from among the above-described examples of the heterocyclic group.

In the present disclosure, the fluorenyl group has a structure in which two cyclic organic compounds are linked to each other by one atom, and examples thereof include

and the like.

In the present disclosure, the fluorenyl groups include an open fluorenyl group having a structure in which one of two cyclic compounds linked to each other by one atom is broken. Examples of the open fluorenyl group include

and the like.

In the present disclosure, the amine group preferably contains 1 to 30 carbon atoms, but is not limited thereto. Specific examples of the amine group include, but are not limited to, methylamine, dimethylamine, ethylamine, diethylamine, phenylamine, naphthylamine, biphenylamine, anthracenylamine, 9-methyl-anthracenylamine, diphenylamine, phenylnaphthylamine, ditolylamine, phenyltolylamine and triphenylamine groups.

In the present disclosure, examples of the arylamine group include substituted or unsubstituted monocyclic diarylamine, substituted or unsubstituted polycyclic diarylamine, and substituted or unsubstituted monocyclic or polycyclic diarylamine groups.

In the present disclosure, the aryl group of the aryloxy, arylthioxy, arylsulfoxy and aralkylamine groups is as defined above for the aryl group.

In the present disclosure, the alkyl group of the alkylthioxy, alkylsulfoxy, alkylamine and aralkylamine groups is as defined above for the alkyl group.

In the present disclosure, the heteroaryl group of the heteroarylamine group may be selected from among the above-described examples of the heterocyclic group.

In the present disclosure, the arylene, alkenylene, fluorenylene, carbazolylene and heteroarylene groups are divalent aryl, alkenyl, fluorenyl and carbazole groups, respectively. These groups are as defined above for the aryl, alkenyl, fluorenyl and carbazole groups, except that they are divalent groups.

As used herein, the term “substituted or unsubstituted” means that it is unsubstituted or substituted with one or more selected from the group consisting of deuterium, a halogen group, an alkyl group, an alkenyl group, an alkoxy group, a cycloalkyl group, a silyl group, an arylalkenyl group, an aryl group, an aryloxy group, an alkylthioxy group, an alkylsulfoxy group, an arylsulfoxy group, a boron group, an alkylamine group, an aralkylamine group, an arylamine group, a heteroaryl group, a carbazole group, an arylamine group, an aryl group, a fluorenyl group, a nitrile group, a nitro group, a hydroxyl group, and a heterocyclic group containing at least one heteroatom selected from among N, O and S.

In one embodiment of the present disclosure, the copolymer comprises one represented by the following formula 4:

wherein l is a mole fraction in the range of 0<l≧1; m is a mole fraction in the range of 0≦m<1; l+m=1; n is an integer ranging from 1 to 10,000; A is represented by formula 1; B, C and D are the same or different and are each independently represented by formula 2 or 3; and at least one of B, C and D is represented by formula 3.

In one embodiment of the present disclosure, A in formula 4 is represented by formula 1, and B is represented by formula 3.

In another embodiment, C in formula 4 is represented by formula 2.

In still another embodiment, D in formula 4 is represented by formula 3.

In yet another embodiment, there is provided a copolymer wherein B and D in formula are represented by formula 3 and C is represented by formula 2.

In one embodiment of the present disclosure, the copolymer may be represented by the following formula 5:

wherein R_(a) to R_(d), R₁ to R₇, o, p, q, r, s and t are as defined in formulas 1 to 3; l is a mole fraction in the range of 0<l≦1; m is a mole fraction in the range of 0≦m<1; l+m=1; and n is an integer ranging from 1 to 10,000.

In one embodiment of the present disclosure, R_(a) and R_(b) are hydrogen.

In another embodiment, R_(c) and R_(d) are hydrogen.

In still another embodiment, s is 1.

In still another embodiment, t is 1.

In still another embodiment, R₄ is hydrogen.

In still another embodiment, R₅ is hydrogen.

In yet another embodiment, R₆ is hydrogen.

In another further embodiment, R₇ is hydrogen.

In one embodiment of the present disclosure, the copolymer may further comprise a fourth unit represented by the following formula 6 and a fifth unit represented by the following formula 7:

wherein R₁ is as defined in formula 1; R₂ and R₃ are as defined in formula 2; l is a mole fraction in the range of 0<l≦1; m is a mole fraction in the range of 0≦m<1; and l+m=1.

In one embodiment of the present disclosure, the end group of the copolymer is selected from the group consisting of hydrogen, deuterium, a halogen group, a nitrile group, a nitro group, an imide group, an amide group, a hydroxyl group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted aryloxy group, a substituted or unsubstituted alkylthioxy group, a substituted or unsubstituted arylthioxy group, a substituted or unsubstituted alkylsulfoxy group, a substituted or unsubstituted arylsulfoxy group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted silyl group, a substituted or unsubstituted boron group, a substituted or unsubstituted alkylamine group, a substituted or unsubstituted aralkylamine group, a substituted or unsubstituted arylamine group, a substituted or unsubstituted heteroarylamine group, a substituted or unsubstituted aryl group, a substituted or unsubstituted fluorenyl group, a substituted or unsubstituted carbazole group, and a substituted or unsubstituted heterocyclic group containing at least one heteroatom selected from among N, O and S.

In one embodiment of the present disclosure, the end group of the copolymer is a heterocyclic group.

In another embodiment of the present disclosure, the end group of the copolymer is an aryl group.

In one embodiment of the present disclosure, the copolymer preferably has a number-average molecular weight of 500-1,000,000 g/mol, and more preferably 10,000-100,000 g/mol.

In one embodiment of the present disclosure, the copolymer may have a molecular weight distribution of 1-100. Preferably, the copolymer has a molecular weight distribution of 1-3.

As the molecular weight distribution decreases and the number-average molecular weight increases, the electrical and mechanical properties of the copolymer improve.

In addition, in order for the copolymer to have a specific level or higher of solubility so that a solution application method is advantageously applied, the copolymer preferably has a number-average molecular weight of 100,000 or less.

The copolymer comprising the unit of formula 1 can be prepared based on Preparation Examples as described below.

The copolymer according to the present disclosure can be prepared by a multi-step chemical reaction. Specifically, monomers can be prepared by an alkylation reaction, the Grignard reaction, the Suzuki coupling reaction and the Stille coupling reaction, after final polymers can be prepared from the monomers by a carbon-carbon coupling reaction such as the Stille coupling reaction. When the substituent to be introduced is a boronic acid or boronic ester compound, the copolymer can be prepared by the Suzuki coupling reaction, and when the substituent to be introduced is a tributyltin compound, the copolymer can be prepared by the Stille coupling reaction, but is not limited thereto.

In another aspect, the present disclosure provides an organic solar cell comprising the copolymer containing the units of formula 1, formula 2 and formula 3.

Specifically, the present disclosure provides an organic solar cell comprising: a first electrode; a second electrode opposite the first electrode; and one or more organic material layers interposed between the first electrode and the second electrode and including a photoactive layer, wherein one or more of the organic material layers comprise the above-described copolymer.

In one embodiment of the present disclosure, the organic solar cell comprises the first electrode, the photoactive layer and the second electrode. The organic solar cell may further comprise a substrate, a hole transport layer and/or an electron transport layer.

The substrate may be a glass or transparent substrate having excellent transparency, surface smoothness, ease of handling, and water resistance, but is not limited thereto.

The substrate is not limited as long as it is commonly used in organic solar cells. Specific examples of the substrate include, but are not limited to, polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polypropylene (PP), polyimide (PI) and triacetyl cellulose (TAC).

The first electrode may be made of a transparent, highly conductive material, but is not limited thereto. Specific examples of the material of the first electrode include, but are not limited to, indium tin oxide (ITO), tin oxide (SnO₂) and zinc oxide (ZnO).

The second electrode may be made of a metal having a low work function, but is not limited thereto. Specific examples of the material of the second electrode include, but are not limited to, metals, such as lithium, magnesium or aluminum, alloys thereof, and multilayer materials, such as Al:Li, Al:BaF₂, or Al:BaF₂:Ba.

The hole transport layer and/or the electron transport layer may be made of a material which efficiently transfers electrons and holes to the photoactive layer to increase the mobility of produced charges to the electrodes, but is not limited thereto.

Examples of the material of the hole transport layer incluce PEDOT:PSS (poly(3,4-ethylenediocythiophene) doped with poly(styrenesulfonic acid)), and N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine (TPD). Examples of the material of the electron transport layer include aluminum trihydroxyquinoline (Alq₃), the 1,3,4-oxadiazole derivative PBD (2-(4-bipheyl)-5-phenyl-1,3,4-oxadiazole), the quinoxaline derivative TPQ (1,3,4-tris[(3-phenyl-6-trifluoromethyl)qunoxaline-2-yl]benzene), and triazole derivatives.

The photoactive layer may comprise an electron donor material and an electron acceptor material.

In one embodiment of the present disclosure, the electron donor material is the copolymer comprising the first unit of formula 1, the second unit of formula 2 and the third unit of formula 3.

In one embodiment of the present disclosure, the electron acceptor material may be fullerene, a fullerene derivative, vasocuproin, a semiconductor element, a semiconductor compound, or a combination of two or more thereof. Specifically, the electron acceptor material may be phenyl C₆₁-butyric acid methyl ester (PC₆₁BM) or phenyl C₇₁-butyric acid methyl ester (PC₇₁BM).

The electron donor material and the electron acceptor material in the photoactive layer can form a bulk heterojunciton (BHJ). The electron donor material and the electron acceptor material may be mixed with each other at a ratio of 1:10-10:1 (w/w). After the electron donor material and the electron acceptor material have been mixed, they may be annealed at a temperature of 30 to 300° C. for 1 second to 24 hours in order to maximize the characteristics thereof.

The thickness of the photoactive layer may be 10-10,000 Å, but is not limited thereto.

In one embodiment of the present disclosure, a buffer layer may further be introduced between the photoactive layer and the first electrode, and an electron transfer layer, a hole blocking layer or an optical space layer may further be introduced between the photoactive layer and the second electrode.

As described in Korean Patent Laid-Open Publication No. 10-2010-0111767 which is incorporated herein by reference, in one preferred embodiment, the organic solar cell comprises the substrate, the first electrode, the photoactive layer and the second electrode. Herein, the photoactive layer comprises an electrode donor made of the copolymer, which comprises the first unit of formula 1, the second unit of formula 2 and the third unit of formula 3, and an electron acceptor made of a C60 fullerene derivative or a C70 fullerene derivative.

In one embodiment of the present disclosure, the photovoltaic material of the photoactive layer comprises the copolymer comprising the first unit of formula 1, the second unit of formula 2 and the third unit of formula 3, and the electron acceptor, which are mixed with each other at a weight ratio of 1:0.5-1:4.

In another embodiment of the present disclosure, the photovoltaic material of the photoactive layer comprises the copolymer comprising the first unit of formula 1, the second unit of formula 2 and the third unit of formula 3, and a C60 fullerene derivative or a C70 fullerene derivative, which are mixed with each other at a weight ratio of 1:0.5-1:4.

If the fullerene derivative is used at a weight ratio of less than 0.5 relative to the copolymer comprising the first unit of formula 1, the second unit of formula 2 and the third unit of formula 3, the mobility of electrons will be insufficient, because the content of the crystallized fullerene derivative will be insufficient, and if the fullerene derivative is used at a weight ratio of more than 4, the efficient absorption of light into the photoactive layer will not be achieved, because the amount of the copolymer comprising the first unit of formula 1, the second unit of formula 2 and the third unit of formula 3 is relatively reduced.

In one embodiment of the present disclosure, the organic solar cell may either comprise an anode, a photoactive electrode and a cathode, which are arranged in that order, or comprise a cathode, a photoactive layer and an anode, which are arranged in that order, but the scope of the present disclosure is not limited thereto.

In another embodiment of the present disclosure, the organic solar cell may either comprise an anode, a hole transport layer, a photoactive layer, an electron transport layer and a cathode, which are arranged in that order, or comprise a cathode, an electron transport layer, a photoactive layer, a hole transport layer and an anode, which are arranged in that order, but the scope of the present disclosure is not limited thereto.

The organic solar cell of the present disclosure can be fabricated using the same materials and method as known in the art, except that the organic material layers comprise the copolymer of the present disclosure.

In another aspect, the present disclosure provides a method for fabricating an organic solar cell, the method comprising the steps of: preparing a substrate; forming a first electrode on a region of rear sides of the substrate; forming on the first layer an organic material layer comprising the above-described copolymer; and forming a second electrode on the organic material layer.

The organic solar cell of the present disclosure can be fabricated, for example, by sequentially depositing the first electrode, the organic material layer and the second electrode on the substrate. Herein, the deposition can be performed using wet coating methods, including gravure printing, offset printing, screen printing, inkjet printing, spin coating, and spray coating, but is not limited thereto.

In the organic solar cell comprising the copolymer including the first unit of formula 1, the second unit of formula 2 and the third unit of formula 3, the copolymer can serve as an electron donor.

Hereinafter, methods for preparing the first unit of formula 1, the second unit of formula 2 and the third unit of formula 3, and a method for fabricating an organic solar cell using these units will be described in detail with reference to examples. It is to be understood, however, that these examples are for illustrative purposes only and are not intended to limit the scope of the present invention.

Example 1 Synthesis of Monomers Example 1-A Preparation of N-9′-heptadecanyl-3,6-dibromocarbazole

In this Example, 9-heptadecane p-toluenesulfonate was prepared with reference to the literature (B. Nicolas, M. Alexandre, M. Leclerc, Adv. Mater. 19, 2295-2300).

3,6-Dibromo-9H-carbazole (4.00 g, 12.3 mmol) and potassium hydroxide (6.90 g, 123 mmol) were added to 30 ml of anhydrous dimethyl sulfoxide and stirred for 1 hour. Then, 9-heptadecane p-toluenesulfonate (7.58 g, 18.5 mmol) was added thereto and stirred for 12 hours.

The mixture was cooled to room temperature, extracted with hexane/ethyl acetate (3:1 v/v) and washed with water, after which the solution was dried with magnesium sulfate (MgSO₄) to remove water. The solvent was removed under reduced pressure, and the residue was purified by silica gel column chromatography to afford a white solid.

Yield: 86%

¹H NMR: (300 MHz, CDCl₃, ppm): δ 8.16 (br, 2H); 7.48 (br, 4H); 4.47 (br, 1H); 2.23 (br, 2H); 1.90 (br, 2H); 1.11 (br, 24H); 0.83 (t, J=7.1 Hz, 6H)

¹³C NMR (75 MHz, CDCl₃, ppm): δ 129.39; 128.93; 123.73; 123.40; 113.42; 110.94; 57.23; 34.03; 32.13; 29.68; 29.66; 29.50; 27.02; 22.99; 14.47

m.p.: 53.6 to 55.7° C.

HRMS (EI⁺, m/z) [M]⁺ calculated for C₂₉H₄₁Br₂N: 561.1606; found: 561.1608.

Example 1-B Preparation of 3,6-bis(4′,4′,5′,5′-tetramethyl-1′,3′,2′-dioxaborolan-2′-yl)-N-9′-heptadecanylcarbazole

N-9′-Heptadecanyl-3,6-dibromocarbazole (4.00 g, 7.10 mmol) was added to and dissolved in 30 ml of anhydrous tetrahydrofuran) and cooled to −78° C., and 1.7 M n-butyllithium in hexane (17.5 ml, 29.8 mmol) was added thereto. The mixture was stirred for 1 hour, and then 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (5.28 g, 28.4 mmol) was added at a time thereto. Then, the mixture was warmed to room temperature and stirred for 12 hours.

Water was added to the mixture to stop the reaction, and the mixture was extracted with diethyl ether and washed with water, after which the solution was dried with magnesium sulfate (MgSO₄) to remove water. The solvent was removed under reduced pressure, and the residue was purified by silica gel column chromatography to afford a white solid.

Yield: 57%

¹H NMR (300 MHz, CDCl₃, ppm): δ 8.68 (br, 2H); 7.87 (br, 2H); 7.56 (br, 1H); 7.39 (br, 1H); 4.58 (br, 1H); 2.29 (br, 2H); 1.90 (br, 2H); 1.39 (br, 24H); 1.13 (br, 24H); 0.81 (t, J=6.8 Hz, 6H)

¹³C NMR (75 MHz, CDCl₃, ppm): δ 144.61; 141.20; 132.08; 128.40; 124.25; 122.85; 111.39; 108.66; 83.90; 56.94; 34.10; 32.14; 29.68; 27.10; 25.36; 22.99; 14.47

m.p.: 147.8˜149.2° C.

HRMS (EI⁺, m/z) [M]⁺ calculated for C₂₉H₄₁Br₂N: 567.5100; found: 567.5103

Example 2 Synthesis of Copolymer 1

3,6-Bis(4′,4′,5′,5′-tetramethyl-1′,3′,2′-dioxaborolan-2′-yl)-N-9′-heptadecanylcarbazole (4.1 mg, 0.006 mmol), 2,7-bis(4′,4′,5′,5′-tetramethyl-1′,3′,2′-dioxaborolan-2′-yl)-9,9-dioctylfluorene (197 mg, 0.306 mmol), 4,7-bis-(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (143 mg, 0.312 mmol), a 20% aqueous solution of tetraethylammonium hydroxide (3 ml), tetrakis(triphenylphosphine)palladium (0) (5 mg), Aliquat 336 were added to 3 ml of toluene and stirred under reflux. After 72 hours, phenylboronic acid (0.05 g) was added to and reacted with the reaction mixture for 3 hours, after which bromobenzene (0.12 g) was added to and reacted with the mixture for 4 hours. Then, the mixture was cooled to room temperature and poured into methanol. The solid was filtered and Soxhlet-extracted with acetone, hexane and chloroform. The extract was precipitated in methanol, and the solid was filtered.

Yield: 61%

Number-average molecular weight: 11,400 g/mol

Weight-average molecular weight: 42,900 g/mol

Example 3 Synthesis of Copolymer 2

3,6-Bis(4′,4′,5′,5′-tetramethyl-1′,3′,2′-dioxaborolan-2′-yl)-N-9′-heptadecanylcarbazole (21 mg, 0.03 mmol), 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (161 mg, 0.28 mmol), 4,7-bis-(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (143 mg, 0.31 mmol), a 20% aqueous solution of tetraethylammonium hydroxide (3 ml), tetrakis(triphenylphosphine)palladium (0) (5 mg) and Aliquat 336 were added to 3 ml of toluene and stirred under reflux. After 72 hours, phenylboronic acid (0.05 g) was added to and reacted with the reaction mixture for 3 hours, after which bromobenzene (0.12 g) was added to and reacted with the mixture for 4 hours. Then, the mixture was cooled to room temperature and poured into methanol, and the solid was filtered and Soxhlet extracted with acetone, hexane and chloroform. The extract was precipitated in methanol, and the solid was filtered.

Yield: 54%

Number-average molecular weight: 14,500 g/mol

Weight-average molecular weight: 42,100 g/mol

Example 4 Synthesis of Copolymer 3

3,6-Bis(4′,4′,5′,5′-tetramethyl-1′,3′,2′-dioxaborolan-2′-yl)-N-9′-heptadecanylcarbazole (42 mg, 0.064 mmol), 2,7-bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (147 mg, 0.256 mmol), 4,7-bis-(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (143 mg, 0.31 mmol), a 20% aqueous solution of tetraethylammonium hydroxide (3 ml), tetrakis(triphenylphosphine)palladium (0) (5 mg) and Aliquat 336 were added to 3 ml of toluene and stirred under reflux. After 72 hours, phenylboronic acid (0.05 g) was added to and reacted with the reaction mixture for 3 hours, after which bromobenzene (0.12 g) was added to and reacted with the mixture for 4 hours. Then, the mixture was cooled to room temperature and poured into methanol, and the solid was filtered and Soxhlet-extracted with acetone, hexane and chloroform. The extract was precipitated in methanol, and the solid was filtered.

Yield: 61%

Number-average molecular weight: 6,000 g/mol

Weight-average molecular weight: 9,200 g/mol

Comparative Example 1 Synthesis of Copolymer 4

2,7-Bis(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9,9-dioctylfluorene (200 mg, 0.32 mmol), 4,7-bis-(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (143 mg, 0.31 mmol), a 20% aqueous solution of tetraethylammonium hydroxide (3 ml), tetrakis(triphenylphosphine)palladium (0) (5 mg) and Aliquat 336 were added to 3 ml of toluene and stirred under reflux. 72 hours, phenylboronic acid (0.05 g) was added to and reacted with the reaction mixture for 3 hours, after which bromobenzene (0.12 g) was added to and reacted with the mixture for 4 hours. Then, the mixture was cooled to room temperature and poured into methanol, and the solid was filtered and Soxhlet-extracted with acetone, hexane and chloroform. The extract was precipitated in methanol, and the solid was filtered.

Yield: 55%

Number-average molecular weight: 18,400 g/mol

Weight-average molecular weight: 43,400 g/mol

Comparative Example 2 Synthesis of Copolymer 5

3,6-Bis(4′,4′,5′,5′-tetramethyl-1′,3′,2′-dioxaborolan-2′-yl)-N-9′-heptadecanylcarbazole (200 mg, 0.304 mmol), 4,7-bis-(5-bromo-2-thienyl)-2,1,3-benzothiadiazole (139 mg, 0.304 mmol), a 20% aqueous solution of tetraethylammonium hydroxide solution (3 ml), tetrakis(triphenylphosphine)palladium (0) (5 mg) and Aliquat 336 were added to 3 ml of toluene for 72 hours. After 72 hours, phenylboronic acid (0.05 g) was added to and reacted with the reaction mixture for 3 hours, after which bromobenzene (0.12 g) was added to and mixed with the mixture for 4 hours. Then, the mixture was cooled to room temperature and poured into methanol, and the solid was filtered and Soxhlet-extracted with acetone, hexane and chloroform. The extract was precipitated in methanol, and the solid was filtered.

Yield: 42%

Number-average molecular weight: 4,500 g/mol

Weight-average molecular weight: 6,100 g/mol

Fabricate of Organic Solar Cells and Measurement of Properties Thereof

Each of the above-prepared copolymers and PC₇₁BM ([6,6]-phenyl C₆₁ butyric acid methyl ester were dissolved in 1,2-dichlorobenzene (DCB) to prepare a composite solution. Herein, the concentration of the copolymer in the composite solution was adjusted to 1-2 wt %. In order to fabricate organic solar cells having a structure of ITO/PEDOT:PSS/photoactive layer/LiF/Al, a glass substrate coated with ITO was ultrasonically washed with distilled water, acetone and 2-propanol, and the surface of the ITO was treated with ozone for 10 minutes, and then spin-coated with PEDOT:PSS (AI 4083) to a thickness of 25 nm, followed by heat treatment at 235° C. for 10 minutes. For coating of the photoactive layer, each of the polymer-PC₇₁BM composite solutions was filtered with a 0.45-μm PP syringe filter and spin-coated on the PEDOT:PSS layer, followed by heat treatment at 120° C. for 5 minutes. Then, LiF was deposited on the photoactive layer to a thickness of 6 Å under a vacuum of 3×10⁻⁸ torr using a thermal evaporator, after which Al was deposited on the LiF to a thickness of 120 nm. The photovoltaic properties of the organic solar cells fabricated as described above were measured under a condition of 100 mW/cm² (AM 1.5), and the results of the measurement are shown in Table 1 below.

TABLE 1 Properties of organic solar cells short open power circuit cir- con- Molar current cuit version ratio (%) density volt- effi- of 3,6- (mA/ age Fill ciency Copolymer carbazole cm2) (V) factor (%) Example 2 Copolymer 1 1 11.6 1.00 42.4 4.9 Example 3 Copolymer 2 5 11.8 1.00 43.1 5.1 Example 4 Copolymer 3 10 9.8 0.98 44.3 4.3 Comparative Copolymer 4 0 8.7 0.85 35.4 2.6 Example 1 Comparative Copolymer 5 100 1.8 0.50 23.6 0.2 Example 2

FIG. 2 is a graphic diagram showing current-voltage curves for the organic solar cells.

As can be seen from the results in Table 1 above, the organic solar cells of Examples 2 to 4, which comprise the copolymer including the first unit of formula 1, had increased efficiency compared to the organic solar cell of Comparative Example 1, comprising the copolymer which does not include the first unit of formula 1, and the organic solar cell of Comparative Example 2, which comprises the copolymer including only the first unit of formula 1.

As can be seen from the results in Table 1, the short-circuit density, fill factor and energy conversion efficiency of the organic solar cells were influenced by the molar ratio of the first unit of formula 1.

Accordingly, it can be concluded that the mobility of holes in the photoactive layer of the organic solar cell is increased due to the presence of 3,6-carbazole so as to approach the mobility of electrons, thereby increasing the energy conversion efficiency of the organic solar cell.

Fabricate of Devices in which Mobilities of Holes and Electrons are to be Measured, and Measurement of the Mobilities

Devices in which the mobilities of holes and electrons are to be measured were fabricated in the same manner as the organic solar cell devices, except that Pd was deposited in place of Al to form the second electrode.

The I-V characteristics of the devices fabricated as described above were measured in a dark place by an I-V analysis system, and the SCLC mobilities were determined using the following equation 1:

$\begin{matrix} {J = {\frac{9}{8}ɛ_{0}ɛ_{r}\mu_{h}\frac{V^{2}}{L^{3}}}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack \end{matrix}$

wherein J is current density, L is the thickness of the active layer film, μ_(h) is charge mobility, ∈_(r) is the relative dielectric constant of the active layer film, ∈o is vacuum permittivity, and V is internal voltage.

TABLE 2 Mobilities of holes and charges in devices in which the mobilities are to be measured Ratio of elec- tron Molar mobil- ratio ity to (%) Hole Electron hole of 3,6- mobility mobility mobil- Copolymer carbazole (cm² V⁻¹S⁻¹) (cm² V⁻¹S⁻¹) ity Example 2 Copolymer  1 9.4 × 10⁻⁵ 1.7 × 10⁻⁴ 1.8 1 Example 3 Copolymer  5 8.9 × 10⁻⁵ 1.6 × 10⁻⁴ 1.8 2 Example 4 Copolymer 10 6.5 × 10⁻⁶ 1.1 × 10⁻⁴ 16.9 3 Comparative Copolymer  0 2.5 × 10⁻⁶ 3.8 × 10⁻⁴ 152 Example 1 4

As can be seen in FIGS. 2 to 5, the ratio of electron mobility to hole mobility in the devices comprising the copolymer including the first unit of formula 1 was lower than that in the device comprising the copolymer which does not include the first unit of formula 1. This suggests that the mobility of holes is increased due to the presence of the first unit of formula 1 so as to approach the mobility of electrons. It is well known to those of ordinary skill in the art that, as the mobilities of electrons and holes in organic solar cells approach each other, the energy conversion efficiency of the organic solar cells increases.

As described above, the copolymer of the present disclosure can be used as a material for the organic material layer of an organic solar cell, and an organic solar cell comprising the same can exhibit excellent properties, including increased open voltage and increased efficiency. In particular, the novel copolymer according to the present disclosure has a deep HOMO level and high charge mobility, and thus can exhibit excellent properties. Furthermore, the copolymer according to the present disclosure may be used in a pure or impure form in an organic solar cell and may be applied using a solution application method. In addition, the copolymer of the present disclosure can increase the optical efficiency of the organic solar cell and improve the cycle life characteristics of the organic solar cell thanks to its thermal stability. 

1. A copolymer comprising a first unit represented by the following formula 1, a second unit represented by the following formula 2 and a third unit represented by the following formula 3:

wherein o, p, q and r are each an integer ranging from 0 to 3; s and t are each an integer ranging from 0 to 4; R_(a) to R_(d) are the same or different and are each independently selected from the group consisting of hydrogen, deuterium, a halogen group, a nitrile group, a nitro group, an imide group, an amide group, a hydroxyl group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted aryloxy group, a substituted or unsubstituted alkylthioxy group, a substituted or unsubstituted arylthioxy group, a substituted or unsubstituted alkylsulfoxy group, a substituted or unsubstituted arylsulfoxy group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted silyl group, a substituted or unsubstituted boron group, a substituted or unsubstituted alkylamine group, a substituted or unsubstituted aralkylamine group, a substituted or unsubstituted arylamine group, a substituted or unsubstituted heteroarylamine group, a substituted or unsubstituted aryl group, a substituted or unsubstituted fluorenyl group, a substituted or unsubstituted carbazole group, and a substituted or unsubstituted heterocyclic group containing at least one heteroatom selected from among N, O and S, or adjacent two of R_(a) to R_(d) may form a condensed ring; and R₁ to R₇ are the same or different and are each independently selected from the group consisting of hydrogen, deuterium, a halogen group, a nitrile group, a nitro group, an imide group, an amide group, a hydroxyl group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted cycloalkyl group, a substituted or unsubstituted alkoxy group, a substituted or unsubstituted aryloxy group, a substituted or unsubstituted alkylthioxy group, a substituted or unsubstituted arylthioxy group, a substituted or unsubstituted alkylsulfoxy group, a substituted or unsubstituted arylsulfoxy group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted silyl group, a substituted or unsubstituted boron group, a substituted or unsubstituted alkylamine group, a substituted or unsubstituted aralkylamine group, a substituted or unsubstituted arylamine group, a substituted or unsubstituted heteroarylamine group, a substituted or unsubstituted aryl group, a substituted or unsubstituted fluorenyl group, a substituted or unsubstituted carbazole group, and a substituted or unsubstituted heterocyclic group containing at least one heteroatom selected from among N, O and S, or adjacent two of R₁ to R₇ may form a condensed ring.
 2. The copolymer of claim 1, wherein the first unit is present in an amount of greater than 0 mole % and 45 mole % or less based on total moles of monomers constituting the copolymer.
 3. The copolymer of claim 1, wherein the copolymer comprises one represented by the following formula 4:

wherein l is a mole fraction in the range of 0<l≦1; m is a mole fraction in the range of 0≦m<1; l+m=1; n is an integer ranging from 1 to 10,000; A is represented by formula 1; B, C and D are the same or different and are each independently represented by formula 2 or 3; and at least one of B, C and D is represented by formula
 3. 4. The copolymer of claim 3, wherein B and D in formula 4 are represented by formula 3, and C is represented by formula
 2. 5. The copolymer of claim 1, wherein the copolymer is represented by the following formula 5:

wherein R_(a) to R_(d), R₁ to R₇, o, p, q, r, s and t are as defined in formulas 1 to 3; l is a mole fraction in the range of 0<l≦1; m is a mole fraction in the range of 0≦m<1; l+m=1; and n is an integer ranging from 1 to 10,000.
 6. The copolymer of claim 1, wherein the copolymer further comprises a fourth unit represented by the following formula 6 and a fifth unit represented by the following formula 7:

wherein R₁ is as defined in formula 1; R₂ and R₃ are as defined in formula 2; l is a mole fraction in the range of 0<l≦1; m is a mole fraction in the range of 0≦m<1; and l+m=1.
 7. The copolymer of claim 1, wherein the copolymer has an aryl group as its end group.
 8. The copolymer of claim 1, wherein the copolymer has a heterocyclic group as its end group.
 9. The copolymer of claim 1, wherein the copolymer has a number-average molecular weight of 500-1,000,000 g/mol.
 10. The copolymer of claim 1, wherein the copolymer has a molecular weight distribution of 1-100.
 11. An organic solar cell comprising: a first electrode; a second electrode opposite the first electrode; and one or more organic material layers interposed between the first electrode and the second electrode and including a photoactive layer, wherein one or more of the organic material layers comprise the copolymer of claim
 1. 12. The organic solar cell of claim 11, wherein the organic material layers comprise one or more selected from the group consisting of an electron donor material and an electron acceptor material, wherein the electron donor is the copolymer.
 13. The organic solar cell of claim 12, wherein the electron acceptor is selected from the group consisting of fullerene, fullerene derivatives, carbon nanotubes, carbon nanotube derivatives, vasocuproin, semiconductor elements, semiconductor compounds, and combinations thereof.
 14. The organic solar cell of claim 12, wherein the electron acceptor is a C60 fullerene derivative or a C70 fullerene derivative.
 15. The organic solar cell of claim 12, wherein the copolymer and the electron acceptor are mixed with each other at a weight ratio of 1:0.5-1:4.
 16. The organic solar cell of claim 14, wherein the polymer and the C60 fullerene derivative or the C70 fullerene derivative are mixed with each other at a weight ratio of 1:0.5-1:4.
 17. A method for fabricating an organic solar cell, the method comprising the steps of: preparing a substrate; forming a first electrode on a region of rear sides of the substrate; forming on the first electrode an organic material layer comprising the copolymer of claim 1; and forming a second electrode on the organic material layer. 